Sodexpert: a secondary-system chemistry expert system in the control room feedback of a test in operating conditions

Operational experience on pressurized water reactors (PWRs) throughout the world, including at EDF, has confirmed the importance of monitoring and controlling secondary-circuit chemistry to protect steam generators (SG) against risks of corrosion. Consequently, EDF was led to impose particularly strict operating specifications to guarantee that the water in SGs was always as pure as possible. These specifications applied at several points on the circuit where the secondary fluid is continuously analyzed. The main specification concerns the cationic conductivity and sodium concentration at the SG blowdown. Various operating zones, some with time limits depending on the pollution concentrations reached, are imposed. Instructions state that, if some of these limits are exceeded, the load must be immediately reduced. However, corrosion problems encountered on the secondary-side of SG tubes, at the tube support plate level, in certain EDF reactors showed that strict compliance with the standards was not enough to avoid this type of corrosion. Consequently, the health physics ALARA principle must be applied to chemistry to protect steam generators against the risk of corrosion due to weak, but long-term pollution. In general, the operator is not capable of this type of ''on-line'' analysis since it requires thorough knowledge of the physico-chemical phenomena involved and because the raw data required to interpret them is fed to a number of recorders whose output is relatively imprecise. To satisfy plant operators' requests, the Laboratories Group, responsible for preparing the Power Generation and Transport Division's (DEPT) policy and the Design and Research Data Processing and Applied Mathematics (IMA) department undertook joint development of the expert system. Work on the product described here began in 1988. (authors). 5 refs